NGC 2403 and NGC 2404 in H-alpha and continuum light

NGC 2403 is a spiral galaxy in Camelopardalis. NGC 2404 is one of many large HII region inside the galaxy.

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NGC 2403 and NGC 2404 in H-alpha and continuum light
(requires JavaScript, H-alpha: on)
Click on the button to switch H-alpha on and off. In both images NIR (near infrared) is mapped to orange and blue is mapped to bluish cyan, but the first image additionally contains H-alpha light in the red channel. (The only difference is that in the H-alpha variant the NIR data in red channel was replaced by data taken with the H-alpha filter which contains about the same continuum information as the NIR data.)

By toggling between the images it can be seen that the HII regions correlate with bluish regions. That's because the HII gas clouds typically contain many young (blue) stars. Furthermore the HII regions probably also emit [OIII] light which is collected by the blue filter.

NGC4504 is the large HII region left to the galaxy center and north of (above) a bright star.

Image data

FOV: 0.47° × 0.40°
Date: 2019-2021
Location: Pulsnitz, Germany
Instrument: 400mm Newton at f=1520mm
Camera Sensor: Panasonic MN34230
Orientation: North is up (approximately)
Scale: 0.8 arcsec/pixel (at full resolution)
Total exposure times:
H-alpha (3nm): 18.0 h
NIR: 3.5 h
Blue: 3.1 h

Image processing

All image processing steps are deterministic, i.e. there was no manual retouching or any other kind of non-reproducible adjustment. The software which was used can be downloaded here.

Image processing steps where:

  1. Bias correction, photon counting
  2. Dark current subtraction, flatfield correction, noise estimation
  3. Alignment and brightness calibration using stars from reference image
  4. Stacking with masking unlikely values and background correction
  5. Denoising and deconvolution
  6. RGB-composition
  7. Dynamic range compression using non-linear high-pass filter
  8. Tonal curve correction

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